1. Field of the Invention
The present invention concerns the detection and localization of hovering helicopters, where this detection has to be done in a short period of time starting from the instant when the helicopters come into view from behind a screen of vegetation.
2. Description of the Prior Art
It is known that a helicopter ca be identified by the echos from its rotor, which are called blade flashes. These echos have certain specific characteristics, notably a wide frequency spectrum due to the Doppler effect on the rotating blades of the rotor and a modulation of amplitude due to the periodic passing of the blades through the perpendicular to the radar-helicopter axis. These characteristics may be exploited to distinguish the helicopters from other targets.
There is, for example, a radar known from the U.S. Pat. No. 4,346,382, which works at low frequency in the 160 MHz band, where the blade flashes intersect each other. In this type of radar, for each distance window, the module of the echo signal is extracted after a synchronous demodulation in phase and in quadrature and a Doppler filtering that eliminates the echo components due to fixed or slow-moving targets, and said module is subjected to a high-pass filtering to extract the amplitude modulation therefrom and trigger a threshold circuit.
This type of radar has the drawback of having a bulky antenna given its low frequency of operation, and of consequently having little mobility.
The present invention is aimed at overcoming this drawback and, more generally, at enabling the detection and localization of helicopters by means of radars working in higher frequency bands that range from 300 MHz to 20 GHz.